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post #1 of 48 (permalink) Old 04-14-2013, 08:15 PM Thread Starter
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Default Tolerances in woodworking

I ran into this thread on another forum I visit where the topic is acceptable tolerances in an aluminum dog manufactured to fit into a 20mm hole arranged on 96mm centers(unlike Harry, I'm bilingual when it comes to measurements ).

I really liked the manufacturer's comments in a reply:

"The talk of tolerances comes up often on this forum and I think it needs some clarifying. Hopefully no one here is trying to make a 3 foot square panel and hold it to .005" tolerance, it just can't be done on a repeatable basis. I would think anything beyond 1/64" (.016") is pushing your luck (this is mostly due to the machines we use, not the wood). But if you're making a dado for a panel to fit in, then 1/64" is very loose and most would find unacceptable for gluing. If you were to make a 1/4" bolt hole in the panel, you may not be happy with 1/64" tolerance."

"When it comes to tolerance for being square, saying it's out by 1/8" doesn't tell anybody anything. We need a length to go with that 1/8". There's a big difference between being out of square by 1/8 over a 12" length and by 1/8" over an 8 foot length. You should always include a length."

"[start rant]
Since World War I (nearly 100 years ago) machinists have understood the need for standards and tolerances. Because of this parts can made in separate parts of the country and still be assembled together with exacting fits. Yet woodworkers continue to buy rulers that are not marked correctly and squares that are not square (would you buy a saw that could not saw???) and it is all done with the excuse that tolerances don't matter with wood because wood moves. That "excuse" should be the exact reason to demand accuracy, not ignore it. Think of all the work that is redone by woodworkers because one guy's ruler was off from the other guys, or something was built to fit a square corner but at install time they discover the corner wasn't square. All of this can easily be stopped by demanding better tolerances from your basic tools.

[end of rant] "


I see this all the time on all the woodworking forums I visit, tight tolerances aren't important because "wood moves". No argument with the "wood moves" part, it does and will continue to do so. But it moves consistently in known ways and that movement can be predicted and accounted for in the design stage of any project.

Using that as a justification for buying poor measurement tools to save a few bucks and not spending the time to learn good measurement practices is just a justification for sloppy work, IMHO.

And what about jigs and fixtures? Good MDF is held to very tight tolerances and doesn't move nearly as much as solid-sawn lumber, good plywood is a close second. I have fixtures and jigs made of MDF, Lexan and precision-ground cast aluminum, they're all accurate to at least .001" in 18" and made with ordinary woodworking tools. So don't tell me it can't be done. The wood that comes out of them is equally accurate, as long as everything is held to close tolerances the finished assembly can be expected to come out as designed with no surprises or refitting.

Real-world example, a typical cabinet door 12" wide, 30" tall in a 24" wide cabinet: Let's say my crosscut on the rails is 1/128"(.008") out of square in 2-1/2 inches of width. Now that 1/128" is negligible because wood moves, right? I clamp that rail tight against the 30" stile, that .008" deviation from square is now 12 times that deviation at the end of the stile(30/2.5=12) so now one side of my door is .096"(3/32") out of square. But wait, there's more! The other side of the door is out an equal amount so I now have a door that's racked 3/16". Now I have the task of trimming the doors on each side from nothing to 3/32" just to square them up. So I finally figure out how to do that but now my perfectly sized doors are 3/8"(3/16"x2) short of the cabinet width they're supposed to fit into.

But that's OK. I'll just tell my customer "wood moves". Right.....

Your thoughts?

Bill

Last edited by billg71; 04-14-2013 at 08:17 PM.
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post #2 of 48 (permalink) Old 04-14-2013, 09:04 PM
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I think repeatability is more important than accuracy in most cases. if one rail of a door is significantly different than the other rail, yeah it will be out of square if you perfectly align corners. Usually, it wont matter that much if the rails are 24.000" or 24.015", as long as both are the same.

Inset doors need to be way more accurate than overlay. With overlay, you may not notice even if they are a bit out of of square! So it really depends on the application.

Dovetails, box joints, dadoes, etc, I try to get as accurate as possible. Missing by small amounts can look really bad. The flip side is... get them too exact and assembly becomes an issue.

So, I think it really depends on the specific project.

That said, I cut several pieces of 3/4 plywood to 3 inch lengths the other day in response to a thread on this forum. Using the Incra jig, I was able to get 5 pieces cut at +- .003". Pretty good for most tablesaws. However, I can feel the difference in them when stacked together. Close enough?? Your mileage, application, and opinion may very. Doesn't make either answer right or wrong!

I have found that hand tools are the best choice when I want to make mistakes at a slower rate of speed.

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post #3 of 48 (permalink) Old 04-14-2013, 09:15 PM
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Hi Bill.
Have to agree that wood "moves".
That's why I prefer to use plywood for larger areas.
Also interested in your comment on wood measurement technique, in that, while some measure and scribe their wood with a magnifying glass and knife edge, I am from the old school where my eyes and a pencil width are adequate for almost all of my wood measurements.
Generally more concerned in the setup of my saws where a true right angle is important to make everything fit.
Mark
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post #4 of 48 (permalink) Old 04-15-2013, 12:05 AM
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I am a machinist by trade, woodworking with machines about 4 years.
In the metal working, one important thing, is not to use super precision where its not needed.
An example- two plates held together with four bolts.
Semi precison layout "scribing lines" should get the bolt holes at least .005" from each other, alittle inacuracy using a drill press added- the plates will still bolt together just fine, using a drill 1/32" oversize of the bolt diameter.

The other side of the coin,
An end cap of a power transfer case, lets say a hydraulic pump body.
Along with bolts, there are four dowl pins that have a slip fit for the cap of .001", and a press fit in the body .001".
That becomes a serious matter, where as, not only the locations of the dowl pins becomes important, but also the hole sizes for the press and slip fits for the dowl pins.
There, a more specialized machine is needed, that better not be out very much more then .0001"- one hundred millionths.

Yes, I have seen folks say you dont need better then 1/64" accuracy in woodworking.
But, they never say just for what- a chicken coup, or a fine jewlery box?

At any rate, I do what I can adjusting my woodworking machines for the best I can get. But not fret over the chicken coup projects.
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post #5 of 48 (permalink) Old 04-15-2013, 03:03 AM
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One thing you didn't mention, Bill, is the traditional woodworking way to overcome many fitting issues: instead of spending hours tuning-up cheap tools to make them more accurate it is often just simpler to get out a hand plane and use hand and eye to achieve the result. You can't do that with steel. As you say there is a time and a place for super accuracy - but there is also a time and a place to use hand tools. The pity is that there are a lot of people who call themselves woodworkers and yet never master the true basics

Regards

Phil

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post #6 of 48 (permalink) Old 04-15-2013, 03:25 AM
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Some members of this forum had almost talked me out of the importance and/or value of what Bill has brought up in this thread. Certainly the issue of application is important. In my case, my knowing that I can to close tolerances whether I need them or not in each project im.portant. I put emphasis on "in my case". I for one, like the time I spend on fiddling with the machinces to see how well I can make them perform. To be clear here, I do not mean to imply that I have the skill or knoe how to actually modify the machines, I am referring to learning and/or acquiring the skills to get the most out of them.

It's a personal thing of course. Those of us that do like to deal with getting tolerances as close as we can are not correct and others are wrong and vise a versa. Both schools are right, it's a just a matter of "what rings your bell I suppose". I for one have really enjoyed this thread and am looking forward to reading more posts to it.

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post #7 of 48 (permalink) Old 04-15-2013, 05:48 AM
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Accuracy also depends (IMHO) on the wood you are using. 1/32 in soft pine is not bad but in purple heart or such it is unacceptable. Pine "moves" a lot more than hardwood.
Just me, I guess.
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post #8 of 48 (permalink) Old 04-15-2013, 07:02 AM
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Quote:
Originally Posted by billg71 View Post

Real-world example, a typical cabinet door 12" wide, 30" tall in a 24" wide cabinet: Let's say my crosscut on the rails is 1/128"(.008") out of square in 2-1/2 inches of width. Now that 1/128" is negligible because wood moves, right? I clamp that rail tight against the 30" stile, that .008" deviation from square is now 12 times that deviation at the end of the stile(30/2.5=12) so now one side of my door is .096"(3/32") out of square. But wait, there's more! The other side of the door is out an equal amount so I now have a door that's racked 3/16". Now I have the task of trimming the doors on each side from nothing to 3/32" just to square them up. So I finally figure out how to do that but now my perfectly sized doors are 3/8"(3/16"x2) short of the cabinet width they're supposed to fit into.

But that's OK. I'll just tell my customer "wood moves". Right.....

Your thoughts?

Bill
Uhhh, unless the 2 rails are not the same length, that should not be. You may have a joint that doesn't meet across the width of the rail. But unless clamped and glued out of square, the door should be square! This points out the need for consistency and craftsmanship as opposed to accuracy. There is a difference between precision and accuracy. 12.00003 is quite precise, but may not be a very accurate at all!

The door example calls for a dry fit and check before glueup, The error should be found before there is a need to shave a finished door! As Phil properly points out, at that point a hand plane can easily and quickly fix the "error".

This is one of the problems common in woodworking today. We expect our machines to eliminate the need for craftsmanship. That is only possible to a certain degree. Even if cut on a CNC, if parts are not assembly properly, errors can still occur!

I have found that hand tools are the best choice when I want to make mistakes at a slower rate of speed.

I don't suffer from insanity. I enjoy every minute of it.

Last edited by Dmeadows; 04-15-2013 at 07:19 AM.
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post #9 of 48 (permalink) Old 04-15-2013, 07:20 AM
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Amen, Phil and Duane!
Take a critical look at any good quality, old furniture. No digital or carbide anything used in their production, just a lot of skill and craftsmanship.
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post #10 of 48 (permalink) Old 04-15-2013, 07:22 AM
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Accuracy - tough subject for many wood workers including me who are hobbiests and who do the best they can with the tools they have. If for instance, am doing something that will be a decorative item in the home, try, if making multiple pieces, no more then a thumb feel difference which is in the low thousandths - doubtful if its as much as that .008 or 1/64th. If it's a piece of molding to case a door frame or window to be painted, close enough could be good enough as there is always painters caulk - and no, this is NOT how I want to do that job - and mine is a lot better then the 'pro' who originally molded the house in 1970. Then there is that kitchen that I finished out with stained oak where close enough couldn't work.

My thinking, such as it is, there are places where the tolerance can be as wide as that proverbial barn door, then there is that area where there is virtually none. Only the craftsman can make the determination as to what 'works' where.

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